ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus GmbHGöttingen, Germany10.5194/acp-10-4823-2010In situ measurements of molecular iodine in the marine boundary layer: the link to macroalgae and the implications for O<sub>3</sub>, IO, OIO and NO<sub>x</sub>HuangR.-J.1SeitzK.2BuxmannJ.2PöhlerD.2HornsbyK. E.3CarpenterL. J.3PlattU.2HoffmannT.11Institute of Inorganic and Analytical Chemistry, Johannes Gutenberg-University of Mainz, Duesbergweg 10–14, 55128 Mainz, Germany2Institute of Environmental Physics, University of Heidelberg, Im Neuenheimer Feld 229, 69120 Heidelberg, Germany3Department of Chemistry, University of York, Heslington, York YO10 5DD, UK26052010101048234833This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from http://www.atmos-chem-phys.net/10/4823/2010/acp-10-4823-2010.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/4823/2010/acp-10-4823-2010.pdf

Discrete in situ atmospheric measurements of molecular iodine
(I<sub>2</sub>) were carried out at Mace Head and Mweenish Bay on the west coast
of Ireland using diffusion denuders in combination with a gas
chromatography-mass spectrometry (GC-MS) method. I<sub>2</sub>, IO and OIO were
also measured by long-path differential optical absorption spectroscopy
(LP-DOAS). The simultaneous denuder and LP-DOAS I<sub>2</sub> measurements were
well correlated (<i>R</i><sup>2</sup>=0.80) but the denuder method recorded much higher
concentrations. This can be attributed to the fact that the in situ measurements
were made near to macroalgal sources of I<sub>2</sub> in the intertidal zone,
whereas the LP-DOAS technique provides distance-averaged mixing ratios of an
inhomogeneous distribution along the light-path. The observed mixing ratios
of I<sub>2</sub> at Mweenish Bay were significantly higher than that at Mace Head,
which is consistent with differences in local algal biomass density and
algal species composition. Above algal beds, levels of I<sub>2</sub> were found to
correlate inversely with tidal height and positively with the concentrations
of O<sub>3</sub> in the surrounding air, indicating a role for O<sub>3</sub> in the
production of I<sub>2</sub> from macroalgae, as has been previously suggested from
laboratory studies. However, measurements made ~150 m away from the
algal beds showed a negative correlation between O<sub>3</sub> and I<sub>2</sub> during
both day and night. We interpret these results to indicate that the released
I<sub>2</sub> can also lead to O<sub>3</sub> destruction via the reaction of O<sub>3</sub> with I
atoms that are formed by the photolysis of I<sub>2</sub> during the day and via the
reaction of I<sub>2</sub> with NO<sub>3</sub> radicals at night. The results show that
the concentrations of daytime IO are correlated with the mixing ratios of
I<sub>2</sub>, and suggest that the local algae sources dominate the inorganic
iodine chemistry at Mace Head and Mweenish Bay.